1. Field of the Invention
The present invention relates to an image sensor, and more particularly to an image sensor for a device such as a digital camera module that is installed in portable electronic devices such as mobile phones and PDAs (Personal Digital Assistants).
2. Prior Art
Recently, there have been remarkable improvements in CPU (Central Processing Unit) performance and rapid developments in image processing technology. This has enabled electronic devices, such as mobile phones, PDAs and computers to be provided with excellent digital image processing capability. Furthermore, the transfer of image data between such devices is very popular, which has spurred the development of high quality image sensors used in image pickup apparatuses of these electronic devices. In the case of devices most frequently used for picking up images, the demand for high quality images is greatest.
Referring to FIG. 6, a conventional image sensor includes a cover 10, a microlens array 30, a color filter array 50, a light receiving unit 70 and a signal processing circuit board 90. The cover 10 is located in the front of the image sensor. The cover 10 is made from a sheet of glass or transparent resin, to prevent elements in the image sensor from being damaged by outside forces or contamination. The microlens array 30 includes a plurality of microlenses arranged in a two-dimensional matrix, to increase the amount of light entering the light receiving unit 70. The color filter array 50 typically includes red, green and blue filters (primary color filters) or cyan, magenta and yellow filters (complementary color filters), which are arranged to correspond to pixels of the light receiving unit 70, respectively, to obtain color information. A color image can be obtained by composing all of the output signals from the pixels. The light receiving unit 70 includes a plurality of pixels which are arranged in a two-dimensional array, each of the pixels having a photoelectric conversion element to generate a signal charge corresponding to the intensity of light received by each pixel. The signal processing circuit board 90 drives the pixels and processes the electronic signals produced by the pixels to form image output signals.
When the image sensor picks up an image, light passes through the cover 10 and is focused by the microlens array 30. Then the light passes through the color filter array 50 and enters the light receiving unit 70. The light receiving unit 70 converts the received light signals into the electronic signals, which are processed by the signal processing circuit board 90 to form image output signals.
Digital camera modules installed in portable electronic devices are becoming progressively more miniaturized. The image forming distance may be very short, such that an angle of incidence of peripheral incoming image light is quite large. After being focused by the microlens array 30, incoming light having the largest angle of incidence may be still incident upon an ineffective region of the light receiving unit 70. This results in a decrease in luminance of peripheral regions, which reduces the overall image quality.
Thus, a need exists for an image sensor which overcomes the above-described problems.